ISO/FDIS 21139-22

ISO /TC 42/JWG 27
Secretariat: ANSI
Date: 2025-12-182026-xx
Permanence and durability of commercial prints — —
Part 22:
Backlit display in indoor or shaded outdoor conditions — Light
stability
Permanence et durabilité des impressions commerciales —
Partie 22: Écran rétroéclairé en intérieur ou en extérieur ombragé — Stabilité de la lumière
FDIS stage
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ii
Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
3.1 Symbols and abbreviated terms . 2
3.2 Measures of exposure severity. 2
3.3 Surroundings of backlit display . 4
3.4 Orientations of the backlit print . 5
3.5 Colour evaluation . 6
4 Use profile . 6
4.1 General . 6
4.2 Parameters of backlit display . 6
4.3 Frontside exposure and environmental conditions . 8
4.4 Equivalent test conditions . 8
4.5 Relevance of use . 12
5 Test method. 13
5.1 General . 13
5.2 Sample preparation . 13
5.3 Common test conditions . 14
5.4 Testing of the frontside of the print . 15
5.5 Testing of the backside of the print . 15
6 Measurement . 15
6.1 General . 15
6.2 Measurement conditions . 16
7 Data analysis . 17
7.1 General . 17
7.2 Image quality parameter for data analysis . 17
7.3 Equivalent test conditions . 18
7.4 Estimation of the time to reach a certain change . 18
8 Test report . 18
Annex A (informative) Relative severity of irradiation with a relative spectral distribution . 20
Annex B (informative) Characterization of an example LED light box. 24
Annex C (informative) sRGB test target . 27
Annex D (informative) Overview of test conditions in backlit context . 29
Bibliography . 30

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
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International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 42, Photography.
ThisThe first edition of ISO 21139-22 cancels and replaces the first edition (of ISO/TS 21139-22:2023),,
which has been technically revised.
The main changes are as follows:
— — the Technical Specification was developed into an International Standard.
A list of all parts in the ISO 21139 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
Backlit display of prints is a market segment in the context of commerce (advertisement, brand shops) and
information (maps, directories). In this use profile the backlit prints are irradiated from their frontside and
from their backside, with irradiations on both sides typically differing by their intensity and spectral
distribution.
Backlit display applies with prints on transparent or translucent foils and/or prints on a textile. This document
focusses on LED-based backlit units and provides information about fluorescent-based backlit units for
reference. These backlit displays may be installed indoor or in shaded outdoor display conditions, for
examples backlit display units in shelters and patios. Backlit displays which are subject to solar radiative
heating or precipitation, introducing extensive temperature cycling, are excluded.
Prints on backlit display may fade or otherwise change in appearance due to various environmental stresses,
including irradiation, heat, humidity, atmospheric pollutants, or biological attack, and the combination of
these factors. One of the most critical degradations is light fading caused by intense irradiation from the backlit
unit as well as by the general irradiation from the viewing environment, which may represent various levels
of intensity and spectral distribution, depending on the installation site in a building, near to a window or in a
shaded outdoor display condition.
The irradiation of the backlit print is therefore typically different from its frontside (oriented towards the
viewing environment) compared to its backside (oriented towards the backlit unit).
In this document, the intensity of the irradiation from either frontside or backside is characterized in terms of
illuminance , 𝐸𝐸 , and the accumulated dose by luminous exposure ., 𝐻𝐻 . Spectral distributions, which cause
v v
varying rates of light fading due to their different UV content, are accounted for by a dimensionless factor
called "relative severity.". This factor compares the fading rates of a specific type of irradiation condition to
those under the standardized exposure condition called 'general"general indoor display'display" (according
to ISO 18937-2), both evaluated at the same illuminance ,, 𝐸𝐸 , as explained in Annex AAnnex A.
v
The lighting design of the backlit display unit may cause inhomogeneity of the backside exposure of the print,
which may in turn introduce inhomogeneous patterns of colour fading or discoloration leading to enhanced
visibility of degradation (an example is illustrated in Annex BAnnex B).). The test method described in this
document does not include the assessment of the impact from inhomogeneity of the backside exposure.
This document provides information about the test conditions for colour fading and discoloration applicable
for the different types of display materials, including transparent films or translucent films, papers, and
fabrics. Furthermore, this document gives guidance for estimation of an equivalent luminous exposure for the
intended time of display, acknowledging the limitations of such generic extrapolations. The display use profile
applies for digital and analogue prints.
The test method described in this document does not address the adverse effects of exposure to atmospheric
pollutants, including ozone, and is also limited to the evaluation of colour changes and therefore does not
require specific methods for the evaluation of physical properties, including changes of tensile strength,
cockling etc. In the case that backlit materials are constructed from laminates, the aforementioned factors are
of less importance.
The general concepts for the exposure characterization of prints on a backlit display provided in this document
may also be considered in museum context with details defined by ISO/TS 18950.
v
Permanence and durability of commercial prints — —
Part 22:
Backlit display in indoor or shaded outdoor conditions — Light
stability
1 Scope
This document specifies the test method for light stability measurements of prints on transparent or
translucent foils, transparent or translucent film, and paper or printed on a textile, which are displayed on
backlit units installed in indoor or in shaded outdoor conditions, which are protected against direct
precipitation and radiative heating. Installations of backlit display units in outdoor areas without shading,
which are exposed to direct weathering and/or radiative heating, are excluded.
This document is applicable to the various product classes of “commercial prints” that are suitable for backlit
display. These commercial prints often contain combinations of text, pictorial images and/or artwork.
This document provides guidelines for colour measurements, data analysis, and also provides guidance for
translation of test results into suitable image permanence performance claims considering the variability of
backlit designs and environmental conditions.
This document is applicable to both analogue and digitally printed matter. Methods and principles apply to
both colour and monochrome prints.
NOTE The test method in this document does not address the specific requirements for testing museum backlit
display, however, some of the elements in this test method (such as exposure in both directions) maycan also be
considered in museum context with details defined by ISO/TS 18950.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/CIE 11664--1, Colorimetry — Part 1: CIE standard colorimetric observers
ISO/CIE 11664--2, Colorimetry — Part 2: CIE standard illuminants
ISO/CIE 11664--4, Colorimetry — Part 4: CIE 1976 L*a*b* colour space
th
CIE 015:2018 Colorimetry, 4 Ed.
Commented [eXtyles1]: eXtyles Inline Standards Citation
Match reports that the normative reference "CIE 015:2018"
is not cited in the text.
ISO 18937--1, Imaging materials — Methods for measuring indoor light stability of photographic prints — Part
1: General guidance and requirements
ISO 18937--2, Imaging materials — Methods for measuring indoor light stability of photographic prints — Part
2: Xenon‐arc lamp exposure
ISO/PAS 18940--1, Imaging materials — Image permanence specification of reflection photographic prints for
indoor applications — Part 1: Test methods
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
3.1 Symbols and abbreviated terms
CCT correlated colour temperature
- see International Electrotechnical Vocabulary (IEV) ref: 845-23-068
CIE Commission internationale de l'éclairage
(International Commission on Illumination)
RSD relative spectral distribution,
GI RSD of the test condition “General Indoor display” – see ISO 18937-2
∗
Δ𝐸𝐸 CIELAB colour difference defined in ISO/CIE 11664-4
ab
∗
Δ𝐸𝐸 average of the CIELAB colour differences of the patches of the test target
ab,ave
(vs. initial) as defined in ISO/PAS 18940-1
∗
Δ𝐸𝐸 average of the CIELAB colour differences of the two patches of the test target
ab,wst
(vs. initial) with the highest and the second highest CIELAB colour difference as defined
in ISO/PAS 18940-1.
[3[3]]
∆E00 CIEDE2000 colour difference as defined in ISO/CIE 11664-6
𝐸𝐸 illuminance
v
τ duty cycle
∗
˜ ∗
𝐻𝐻 severity-weighted luminous exposure at which a CIELAB colour change 𝛥𝛥𝐸𝐸 is observed
v,𝛥𝛥𝐸𝐸 ab
ab
3.2 Measures of exposure severity
3.2.1 3.2.1
relative spectral distribution

𝑺𝑺(𝝀𝝀)
quotient of the spectral distribution, 𝑋𝑋 (𝜆𝜆), of a radiant, luminous or photon quantity, 𝑋𝑋(𝜆𝜆), and a fixed
𝜆𝜆
reference value, ,𝑅𝑅, which can be an average value, a maximum value or an arbitrarily chosen value of this
distribution
𝑋𝑋 (𝜆𝜆)
𝜆𝜆
𝑆𝑆(𝜆𝜆) =
𝑅𝑅
Note 1 to entry: The relative spectral distribution has unit one.
[SOURCE: IEV ref: 845-23-068, modified – NOTE 2 to entry has been removed].]
3.2.2 3.2.2
relative severity
𝝆𝝆
𝑺𝑺⁄𝑺𝑺
𝒓𝒓𝒓𝒓𝒓𝒓
ratio of the expected density loss due to light fading for exposure under a given relative spectral distribution,
S(λ) (3.2.1(3.2.1)) in comparison to a reference (𝑆𝑆 (λ) with both exposures at the same level of illuminance
𝑟𝑟𝑟𝑟𝑟𝑟
,, 𝐸𝐸 , as evaluated based by an average action spectrum model (see Annex AAnnex A))
v
Note 1 to entry: For typical relative spectral distributions S(λ) the degree of light fading obtained for the same amount
of luminous energy has been expressed in relative units to each other based on experimental data and an action spectrum
model obtained for typical CMY colorants used in digital printing, see Annex AAnnex A.
Note 2 to entry: Standardized relative spectral distributions S(λ) include daylight filtered Xenon-arc (see ISO 18930),
window-glass filtered Xenon-arc for simulated in-window display or with additional UV blocking for general indoor
display (see ISO 18937-2), fluorescent light (see ISO 18909) and LED light (see ISO 18937-3).
EXAMPLE In this document, 𝜌𝜌 denotes the relative severity of a given relative spectral distribution S(λ)
𝑆𝑆⁄GI
compared to that of the reference exposure test condition “General IndoorGenerali display”.
3.2.3 3.2.3
severity-weighted illuminance
˜
𝑬𝑬
𝒗𝒗,𝑺𝑺/𝑺𝑺
𝒓𝒓𝒓𝒓𝒓𝒓
effective illuminance caused by a light source with a relative spectral distribution, S(λ), (3.2.1(3.2.1)) that is
obtained by multiplying its illuminance , 𝐸𝐸 , with the duty cycle τ (in %) in the application and its relative
v
severity (3.2.2(3.2.2)) in comparison to a continuous exposure with a reference relative spectral distribution
(𝑆𝑆 (λ)
𝑟𝑟𝑟𝑟𝑟𝑟
˜
Note 1 to entry: 𝐸𝐸 = 𝐸𝐸 ⋅𝜏𝜏⋅𝜌𝜌 .
v, 𝑆𝑆/𝑆𝑆 v 𝑆𝑆⁄𝑆𝑆
𝑟𝑟𝑟𝑟𝑟𝑟 𝑟𝑟𝑟𝑟𝑟𝑟
Note 2 to entry: In this test method, the relative spectral distribution of the general indoor filtered Xenon-arc test
method as defined in ISO 18937-2 is used as the reference, so (𝑆𝑆 (λ) = GI. The required spectral irraditionirradiation
𝑟𝑟𝑟𝑟𝑟𝑟
can be achieved by using a window-glass filtered Xe-arc lamp with additional optical filters such as L-37 (Hoya Co.) and
SC-37 (Fujifilm Co.).
Note 3 to entry: The severity-weighted luminous exposures of the frontside and the backside of a print displayed on a
backlit unit typically differ because of different relative spectral distributions, duty cycles and/or intensity.
3.2.4 3.2.4
severity-weighted luminous exposure

˜
𝑯𝑯
𝒗𝒗,𝑺𝑺/𝑺𝑺
𝒓𝒓𝒓𝒓𝒓𝒓
˜
luminous exposure resulting from luminous energy of a severity‐weighted illuminance, 𝐸𝐸 , (3.2.3 (3.2.3))
v, 𝑆𝑆/𝑆𝑆
𝑟𝑟𝑟𝑟𝑟𝑟
that is accumulated over an exposure time, t
exp
Note 1 to entry: In this test document, the relative spectral distribution of the general indoor filtered Xenon-arc test
method as defined in ISO 18937-2 is used as the reference, so (𝑆𝑆 (λ) = GI (“General indoor display”).
𝑟𝑟𝑟𝑟𝑟𝑟
Note 2 to entry: The severity-weighted luminous exposures on the frontside and the backside of a backlit displayed
print, respectively, are typically different and both contribute to colour fading.
3.2.5 3.2.5
UV cut-on
λ
cut-on
wavelength at which the cumulative intensity of a relative spectral distribution, S(λ), (3.2.1(3.2.1)) has reached
0,05 % of its total integrated intensity over the spectral range of 295 nm to 800 nm
𝜆𝜆
cut−on
800𝑛𝑛𝑛𝑛
Note 1 to entry: � 𝑆𝑆(𝜆𝜆)𝑑𝑑𝜆𝜆/ 𝑆𝑆(𝜆𝜆)𝑑𝑑𝜆𝜆 = 0,05%.
∫
295𝑛𝑛𝑛𝑛
295𝑛𝑛𝑛𝑛
3.2.6 3.2.6
λ
50 %
wavelength at which the transmission of an optical long pass filter reaches a value of 50 %.
3.3 Surroundings of backlit display
3.3.1 3.3.1
shaded outdoor display
outdoor mounting display position that is characterized by blocking of both direct sunshine and solar radiative
heating
Note 1 to entry: The UV cut-on (λcut-on) of shaded outdoor display is in the range of 295 nm to 310 nm.
EXAMPLESEXAMPLE Roof-covered or shielded outdoor places, including patios, transportation hubs, such as
airports, train stations, and bus terminals, or the entrance porches of dwelling places, such as entertainment venues,
hotels, and shopping centers.
3.3.2 3.3.2
glass-filtered shaded outdoor display
shaded outdoordisplay (3.3.1outdoor display (3.3.1)) mounting position with optical filtering of the outdoor
irradiation by the front screen of the backlit display unit
Note 1 to entry: Backlit display units in outdoor environments practically always require a screen in front of the print
for reasons of electrical safety. Such a front screen is most often realized by safety glass, such as polyvinyl-butyral (PVB)
laminated sheets of glass or a similar suitable material. The UV cut-on of safety glass varies between 300 nm and 400 nm
depending on its construction and its material formulation. For this standard, 6 mm window glass is defined as reference
for the filter transmission, acknowledging that the UV transmission of different types of front screens varies.
Note 2 to entry: The UV cut‐on (3.2.5(3.2.5)) of glass-filtered daylight is around 320 nm.
3.3.3 3.3.3
in-window display
indoor display mounting position where the irradiation from the surrounding is dominated by terrestrial
daylight transmitted through standard architectural window glass (double glazing)
Note 1 to entry: The UV cut‐on (3.2.5(3.2.5)) of in-window displays is around 320 nm.
EXAMPLESEXAMPLE Mounting positions inside store windows or in other glass-enclosed architectural
constructions (hallways, lobbies, verandas), that face toward the outdoors.
3.3.4 3.3.4
general indoor display
indoor display mounting position where the irradiation from the surrounding is dominated by indirect
lighting, due to filtering (through window glass) and shading
Note 1 to entry: The UV cut‐on (3.2.5(3.2.5)) is around 350 nm.
EXAMPLESEXAMPLE Mounting positions inside a building in distance to windows and/or facing away from
the outdoors.
3.4 Orientations of the backlit print
3.4.1 3.4.1
frontside
face of the backlit print that is intended to be oriented towards the viewer (away from the backlit unit) when
mounted for display
Note 1 to entry: Depending on the print technology the colorants can be deposited on frontside or backside or both
sides which can vary the position of the substrate or any auxiliary layer (pre-white, post-white, laminate) relative to the
colorants and the light source.
Note 2 to entry: Schematics of the elements in backlit display (not to scale):
21139-22_ed1figText1.EPS
Key
1 viewer
2 backlit print
3 backlit unit
4 frontside of the backlit print
5 backside of the backlit print
6 surrounding illumination
7 backlit illumination
3.4.2 3.4.2
backside
face of the backlit print that is oriented towards the backlit unit (away from the viewer) when mounted for
display
Note 1 to entry: Depending on the print technology the colorants can be deposited on frontside or backside or both
sides which can vary the position of the substrate or any auxiliary layer (pre-white, post-white, laminate) relative to the
colorants and the light source.
Note 2 to entry: See frontside (3.4.1(3.4.1),), NOTE 2 to entry.
3.5 Colour evaluation
3.5.1 3.5.1
colorimetric calculation
evaluation of the CIE colour coordinates using the tristimulus values of the minimum density of the backlit
print as the white reference for the calculation of the L*a*b* values.
4 Use profile
4.1 General
This document defines a test method for prints on transparent or translucent foils and/or on textiles that are
displayed on backlit units in indoor or in shaded outdoor display conditions, where the primary stress factors
are exposure to light from both backside and frontside.
NOTE 1 Heat, humidity and atmospheric pollutants can also be stress factors, however this document focuses on light
stability. Heat can have effects on prints that are displayed for long durations on backlit units with elevated temperature,
e.g. due to dissipative heating from electrical appliances in poor-ventilated constructions of the backlit unit itself.
The use profile of commercial prints is described in general in ISO/TS 21139-1. It specifically describes test
methods for backlit display in indoor and in shaded outdoor display conditions, defined as display use profiles
A3 and B1 b) of ISO/TS 21139-1:2019, Table 3, respectively.
NOTE 2 The overall appearance of the displayed prints can also be affected by factors given by the backlit unit itself,
including a non-homogenous distribution of the intensity and/or the correlated colour temperature (CCT) of the backlit
lighting and/or changes of any other element of the backlit unit, e.g. yellowing of the front screen.
4.2 Parameters of backlit display
A backlit display unit is designed to provide a homogenous backside illumination of the print, such that the
brightness of the displayed print is comparable to or larger than the light level of the surrounding viewing
environment. Furthermore, the CCT of the lamps in the backlit unit is often selected to match the viewing
environment, which is typically between 5 000 K and 6 500 K for naturally illuminated areas and between
3 000 K and 4 000 K for some indoor installations.
The spectral irradiation, intensity, and homogeneity of the backside exposure of the print depends on the
construction of the backlit unit. These parameters together with the duty cycle of the backside illumination
determine the severity of the exposure of the print from its backside. Table 1Table 1 provides an overview of
typical parameters associated with LED or fluorescent lamp illuminated light box designs.
The magnitude of temperature increase of the print on backlit display is driven by the dissipative heating from
the backlit lighting system in operation and the degree of air ventilation of the light box in a certain
environment.
NOTE LED back illumination has recently become the dominating technology for new installed backlit units. In this
application, LED lamps replace fluorescent lamps, the use of which is diminished in many countries worldwide because
of environmental concerns and regulations (release of mercury).
The amount of temperature increase is larger in the case of poor air ventilation. Factors that reduce air
ventilation include an airtight design of the housing, its eventual installation onto or especially into a wall, the
use of a front screen and/or the display of a print on a foil (as opposed to a fabric with an open mesh structure).
For heat sensitive materials the temperature increase above the surrounding temperature may have to be
considered.
Table 1 — Parameters of backlit displays
a
Illumination type
Display parameters Glass-filtered
LED Bare-bulb fluorescent
fluorescent
Relative spectral distribution (RSD) S(λ) see ISO 18937-3, phosphor-
converted blue LED See Annex AAnnex A see ISO 18909
(ca. (~5 000 K CCT)
Illuminance level 𝐸𝐸
v
7 to 10
at the backside of the print (in klx)
a
Typical UV content no RSD below 400 nm, but
b
intense blue emission peak mercury lines at 313 nm and 365 nm
around 440 nm to 450 nm
a a a
relative severity 𝜌𝜌 0,73 0,74 0,64
𝑆𝑆⁄GI
Temperature non-ventilated (e.g. front
+7 +15 +15
increase [K] screen and/or foil)
over ambient
ventilated (e.g. open front with
+5 +10 +10
mesh material / /fabric)
Duty-cycle τ (in %) Between “x %” (‘cyclic’) and 100 % (‘24/7’)
a   See Annex AAnnex A.
b   The intensity of the UV lines at 313 nm and 365 nm, that are typically emitted from fluorescent lamps, depends on several
factors, including the amount of mercury used in a specific type of lamp and the level of UV attenuation from the glass envelope of
the lamp and the type and thickness of the phosphor layer. During the use time of the fluorescent lamps, pinholes can be introduced
in the phosphor layer, which can increase the intensity of the UV emission lines over time. On the other hand, the UV lines will be
largely attenuated when an UV absorbing (diffusor) screen is present in between the fluorescent lamps and the print on display.
The glass-filtered fluorescent condition is realized most often, whereas the bare-bulb condition can be regarded as worst case.
For print materials with limited light stability a certain level of inhomogeneity of the backlit illumination (see
example in Annex BAnnex B)) may be sufficient to introduce visible patterns of discoloration. The level of
inhomogeneity of the light intensity, expressed as (E − E )/(E + E ) ⨯ 100 %, may typically range
max min max min
from 10 % to 50 % and stems from the light box design, including:
a) a) the position, geometry, and type of the lighting elements, such as e.g.
1) 1) array of linear lamps, e.g. LED lines, or fluorescent tubes,
2) 2) array of spot lamps, e.g. grid or matrix of individual LED spots, and
3) 3) continuous area illumination, e.g. edge-lit backside diffusor screen;
b) b) the efficiency of the light distribution by the combination of all optical elements:
1) 1) angular emission of the lighting elements, also considering lenses;
2) 2) diffusor-screens;
3) 3) reflectivity of the inner walls.
Also, the reflectivity of the backside of the print itself, when mounted on the backlit unit, may contribute to
the overall system illumination homogeneity.
4.3 Frontside exposure and environmental conditions
The frontside of the print is exposed by the ambient illumination that is present at the installation site of the
backlit display unit. The corresponding environmental parameters may vary between those typical for general
indoor display [A2 of ISO/TS 21139-1:2019, Table 3], or for in-window display [A1 of ISO/TS 21139-1:2019,
Table 3] or for protected outdoor display [B1 b) ISO/TS 21139-1:2019, Table 3]. Users shall identify the most
severe test condition anticipated for their display application and based on that condition estimate a typical
amount of total light exposure during the defined display period. Guidelines are provided in ISO/TS 21139-1
and examples are given further below.
Table 2 — Characterization of typical surroundings of backlit display
glass-filtered
Environmental surrounding general indoor display in-window display
shaded outdoor display
Relative spectral distribution See ISO 18937-2 See ISO 18937-2 See ISO 18937-2
a a
(RSD): S(λ) (General indoor display) (in-window display) (in-window display)
In publicly accessible areas, front screens on the backlit units provide protection
against electrical hazards. These front screens are typically made from safety
glass (see Annex AAnnex A)) or a suitable plastic, and therefore introduce UV-
UV filter function by the front filtering of the surrounding irradiation. Because of front sheet as safety elements,
screen of the backlit unit the in-window-display and glass-filtered shaded outdoor display result in
equivalent severity. Note, that depending on the construction of the backlit unit, a
diffuser screen, or the supporting substrate (film) of the print itself may also act
as UV-filter for the corresponding exposure by the backlit illumination.
b
UV fraction 4 % 6 % 6 %
UV cut-on λcut-on (in nm) 350 320 320
c
c c c
Relative severity 𝜌𝜌
1,0 1,2 1,2
𝑆𝑆⁄GI
d
Duty cycle τ (in %) 50 % to 100 % (‘24/7’) Typically, 50 % Typically, 50 %
a   The test method ‘"in-window display’display" of ISO 18937-2 with continuous light exposure is equivalent to the light stability
test method stipulated in ISO/TS 21139-21.
b   The UV fraction of an RSD S(λ) is indicated as the ratio of the cumulative intensity in the range of 300 nm to 400 nm versus the
cumulative intensity energy in the range of 300 nm to 800 nm (see ISO/TS 21139-1:2019, Annex D). For comparison: natural
daylight has approximately 8 % UV fraction.
c   Reference values from Annex AAnnex A.
d   Duty cycle represents the fraction of time, when the dominating surrounding illumination is present. This can be different for
surroundings with artificial illumination (up to 100 % in case of ‘24/7’) or surroundings where the illumination originates from
daylight (typically, 50 % of daytime).
4.4 Equivalent test conditions
4.4.1 General
In the practical application, any of the combinations of Table 1Table 1 for backside exposure and
Table 2Table 2 for frontside exposure can be observed. To reduce the variability of testing the concept of
equivalent test conditions is applied in this test method and the equivalent luminous exposure is determined.
In this document the concept of “severity-weighted exposure” is applied, which allows one to characterize the
exposure intensity in terms of illuminance (lux), still considering the different UV content of a given RSD. More
background on this approach is given in Annex AAnnex A.
4.4.2 Severity-weighted exposure condition
˜
In a first step, the user of this method needs to determine the severity-weighted illuminance , 𝐸𝐸 , of
v,𝑆𝑆/GI
frontside and backside exposure, as given in Formulae (1)Formulae (1) and (2)(2),, respectively: The
measured illuminance , 𝐸𝐸 , (in klx) is weighted with the duty cycle 𝜏𝜏 of the exposure and the relative severity
v
, 𝜌𝜌 , of the RSD of either side of the print, 𝜌𝜌 or 𝜌𝜌 , respectively. The relative severity , 𝜌𝜌 ,
𝑆𝑆⁄GI 𝑆𝑆 ⁄GI 𝑆𝑆 ⁄GI 𝑆𝑆⁄GI
front back
provides a ratio of degradation expected due to photolytic action of exposure under a given RSD in comparison
to that of the “general indoor display” condition as defined in ISO 18937-2. Annex AAnnex A provides
reference values for the relative severity of several RSDs that have been evaluated on average for typical
colorants based on a general action factor model. As an alternative, the relative severity can be evaluated based
[10 [9]]
on the actually measured spectral action factor of a specific colorant set under investigation, when these
data are available.:
(1)
(2)
with
˜
𝐸𝐸 = 𝐸𝐸 ⋅𝜏𝜏⋅𝜌𝜌 (1)
⁄
v,𝑆𝑆 /GI v 𝑆𝑆 GI
front front
˜
𝐸𝐸 = 𝐸𝐸 ⋅𝜏𝜏⋅𝜌𝜌 (2)
⁄
v,𝑆𝑆 /GI v 𝑆𝑆 GI
back back
where
˜
𝐸𝐸 is the severity-weighted exposure (in klx);
v,𝑆𝑆/GI
𝐸𝐸 is the illuminance (in klx);
v
τ is the duty cycle (in %);
𝜌𝜌 is the relative severity of the RSD S(λ) incident on the frontside or the backside of the print compared to general
𝑆𝑆⁄GI
indoor display – see Annex AAnnex A – (unitless factor).
˜
Table 3Table 3 provides examples of severity-weighted illuminance , 𝐸𝐸 , in standardized exposure
v, 𝑆𝑆/GI
conditions.
In Table 3Table 3,, two main cases are identified:
— — Typically, for glass-filtered shaded outdoor display conditions of backlit, the severity-weighted
exposure of the frontside from the surrounding can be similar or up to 4x larger compared to the severity-
weighted exposure from the backlit unit.
— — For backlit display in indoor conditions, the severity-weighted backside exposure from the backlit unit
is typically considerably larger (up to 35 times) than the severity-weighted frontside exposure from the
surrounding.
The relative severity of a backside illumination by LED and/or by fluorescent lamps is rather comparable (see
Annex AAnnex A).).
NOTE 1 Backlit display units for indoor display of soft signage are often designed without a front screen.
Annex DAnnex D provides an overview of accelerated laboratory test conditions that would potentially
correspond to the exposure conditions in Table 3Table 3:: each test condition is associated to a defined RSD
with a characteristic effective UV cut-on wavelength, for which its relative severity 𝜌𝜌 has been determined
𝑆𝑆⁄GI
(see Annex AAnnex A).).
˜
Table 3 — Examples of the evaluation of severity-weighted illuminance, 𝑬𝑬
𝒗𝒗,𝒔𝒔/𝑮𝑮𝑮𝑮
backside exposure frontside exposure
(from the backlit unit) (from the environment)
#1 #2 #3 #4 #5
LED backlit glass-filtered general indoor in- glass-
(ca. fluorescent indoor window filtered
Exposure conditions
(~5 000 K backlit display display shaded
CCT) (see outdoor
Annex AAnne display
x A))
c a a a a a
Relative severity 𝜌𝜌 P 0,72 0,58 0,87 0,87 0,87
𝑆𝑆⁄GI
Example illuminance 𝐸𝐸
v
10 10 0,5 3 20
d
in use profile (in klx)
Example duty cycle τ 50 % 50 % 50 % 50 % 50 %
a a a
˜
𝐸𝐸 (in klx) 3,6 2,9 0,22 1,3 8,7
v,𝑆𝑆/GI
c
b b b b b
relative severity 𝜌𝜌 P 0,73 0,64 1,0 1,2 1,2
⁄
𝑆𝑆 GI
Example illuminance 𝐸𝐸
v
10 10 0,5 3 20
d
in use profile (in klx)
Example duty cycle τ 100 % 100 % 100 % 50 % 50 %
˜ b b b b b
𝐸𝐸 (in klx) 7,3 6,4 0,5 1,8 12
v,𝑆𝑆/GI
a   Evaluated with a screen between light source and print, that acts as extended UV filter, e.g. λ50 % = 400 nm, typical for UV
stabilized polycarbonate (PC) or poly(methyl methacrylate) (PMMA).
b   Evaluated with RSD typical for use profile, see Table 2Table 2.
c   See Annex AAnnex A.
d   Illustrative setpoints estimated based on ISO/TS 21139-1:2019, Clause 4 and Table 3.
For reasons of practicality and comparability, the test method “general indoor display” of ISO 18937--2 (see
entry #3 in Table D.1Table D.1),), is chosen as thé standard method for the exposure of the frontside and the
backside of the prints in this test method, respectively, see 5.3.15.3.1. The test is conducted at a setpoint of
the illuminance 𝐸𝐸 between 50 klx and 80 klx in the specimen plane.
v,setpoint
NOTE 2 The use of a lower level of the setpoint value results in proportionally longer durations of the accelerated test,
see 4.4.24.4.2.
4.4.3 Equivalent test duration
For the definition of a representative test duration, T, for the frontside and backside exposures, respectively,
a nominal display exposure time , 𝑡𝑡 , is defined. Criteria for the selection of such a representative display
exp
exposure time include worst case scenarios for the anticipated use profile or other criteria agreed upon
between parties. From the nominal display exposure time, ,𝑡𝑡 , the severity-weighted luminous exposure ,
exp
˜
𝐻𝐻 , is calculated for frontside and backside exposure separately as given in Formulae (3)Formulae (3) and
v,𝑆𝑆/GI
(4)(4)::
(3)
(4)
with
˜ ˜
𝐻𝐻 = 𝐸𝐸 ·𝑡𝑡
v,𝑆𝑆 /GI v,𝑆𝑆 /GI exp
front front
MAX MIN
(3)
˜ ˜
𝐻𝐻 = 𝐸𝐸 ·𝑡𝑡 (4)
v,𝑆𝑆 /GI v,𝑆𝑆 /GI exp
back back
where
˜
𝐻𝐻 is the severity-weighted luminous exposure (in Mlx·h) for the RSD incident on the frontside or the backside,
v,𝑆𝑆/GI
respectively;
˜
𝐸𝐸 is the severity-weighted illuminance (in klx).);
v,𝑆𝑆/GI
𝑡𝑡 is the nominal display exposure time defined for the application (in hours).
exp
The corresponding durations, T, of the frontside and backside exposures in the accelerated test, respectively,
˜
are determined as the exposure time needed to provide the equivalent luminous exposure , 𝐻𝐻 , for the test
v,𝑆𝑆/GI
condition with the RSD GI and test illuminance , 𝐸𝐸 , (in klx) at the setpoint SP in the accelerated test, see
v,setpoint
Formulae (5)Formulae (5) and (6)(6)::
Tfront = / = (5)
T = / = (6)
back
˜
Tfront = 𝐻𝐻 /𝐸𝐸 = 𝐸𝐸 ⋅𝜏𝜏⋅𝜌𝜌 ·𝑡𝑡 / 𝐸𝐸 (5)
v,𝑆𝑆 /GI v,setpoint v 𝑆𝑆 ⁄GI exp v,setpoint
front front
˜
Tback = 𝐻𝐻 /𝐸𝐸 = 𝐸𝐸 ⋅𝜏𝜏⋅𝜌𝜌 ·𝑡𝑡 / 𝐸𝐸 (6)
v,𝑆𝑆 /GI v,setpoint v 𝑆𝑆 ⁄GI exp v,setpoint
back back
where
𝑇𝑇 is the duration of the accelerated (in hours));
˜
𝐻𝐻 is the severity-weighted luminous exposure (in Mlx·h) for the RSD incident on the frontside
v,𝑆𝑆/GI
or the backside, respectively;
˜
is the severity-weighted exposure (in klx);
𝐸𝐸
v,𝑆𝑆/GI
𝐸𝐸 is the setpoint of the illuminance (GI) in the accelerated test (in klx));
v,setpoint
𝑡𝑡 is the nominal display exposure time defined for the application (in hours)).
exp
Table 4Table 4 provides illustrative examples of the calculation of the test durations based on
Formulae (1)Formulae (1) to (6)(6) for three display configurations, which are combinations of the exposure
condition at the frontside and the backside, as respectively given in Table 3Table 3.
Table 4 — Examples of test durations T for typical backlit display use profiles
Example 1 Example 2 Example 3
Display configuration backside frontside backside frontside backside frontside
LED backlit general LED backlit in-window LED backlit glass-
(ca. indoor (ca. display, with (ca. filtered
(~5 000 K display, with (~5 000 K a UV (~5 000 K shaded
CCT) no front CCT) filtering CCT) outdoor
screen front screen display
conditions in the application
Relative severity 𝜌𝜌 0,73 1 0,73 0,87 0,73 1,2
⁄
𝑆𝑆 GI
illuminance 𝐸𝐸 (in klx) 10 0,5 10 3 10 20
v
duty cycle τ 50 % 100 % 100 % 50 % 50 % 50 %
Example 1 Example 2 Example 3
Display configuration backside frontside backside frontside backside frontside
LED backlit general LED backlit in-window LED backlit glass-
(ca. indoor (ca. display, with (ca. filtered
(~5 000 K display, with (~5 000 K a UV (~5 000 K shaded
CCT) no front CCT) filtering CCT) outdoor
screen front screen display
˜
𝐸𝐸 (in klx) see
v,𝑆𝑆/GI
3,65 0,5 7,3 1,305 3,65 12
Formulae (1)formulae
(1) and (2)(2)
nominal display
4 320 4 320 2 160 2 160 8 640 8 640
duration 𝑡𝑡 (in h) (180 d) (180 d) (90 d) (90 d) (360 d) (360 d)
exp
˜
𝐻𝐻 (in Mlx·h) – see
v,𝑆𝑆/GI
15,8 2,2 15,8 2,8 31,5 103,7
Formulae (3)formulae
(3) and (4)(4)
˜
duration to obtain the same luminous exposure 𝐻𝐻 with the test condition:
v,𝑆𝑆/GI
RSD = GI, duty cycle τ = 100 % and 𝐸𝐸 = 80 klx
v,setpoint
Exposure side backside frontside backside frontside backside frontside
T (in h) – see
Formulae (5)formulae 197 27 197 35 394 1 296
(
...